Cryocoolers systems are used, for example, to cool infrared sensors during operation. A cryocooler system typically includes a reciprocating compression piston and a reciprocating regenerator/displacer piston. In some cryocooler systems a single rotary motor is used to drive both pistons. Such systems include a first drive coupling disposed between a shaft of the rotary motor and the compression piston and a second drive coupling disposed between the shaft of the rotary motor and the regenerator piston. Rotation of the motor shaft is coupled to each piston thereby reciprocally driving each piston within a drive cylinder. The reciprocating motion of the pistons is out of phase with each other.
It is a conventional problem that the piston drive couplings induce vibrations in the cryocooler system. These vibrations are coupled to the infrared sensor and can degrade image quality. It is particularly problematic when the piston drive couplings excite elements of the cryocooler system at their natural frequency. It is a further problem that the piston drive couplings generate undesirable audible noise. Undesirable vibrations and audible noise are partially caused by excess looseness and also by misalignment of the coupling elements.
To reduce excess play and improve audible noise, it is conventional to tighten coupling element mechanical joint fit tolerances. For example, the drive coupling drives the regenerator piston through a regenerator link that attaches to the drive coupling through a connecting pin. The drive coupling, the regenerator link, and the regenerator piston thus each have corresponding bearings to receive the connecting pins. The clearance between the connecting pin bearings and the connecting pins represents a common type of mechanical joint fit tolerance that is tightened to reduce excess play and noise. However, as this clearance is reduced towards zero, the ever tighter mechanical coupling leads to regenerator link failure due to high stresses induced by misalignment leading to bending stresses. Such a close tolerance may cause the cooler to operate at maximum input power and maximum rpm, leading to accelerated failure of other moving parts such as ball bearing, linkages and related components. In particular, small misalignments between the motor drive shaft longitudinal axis and the regenerator piston longitudinal axis (ideally, the alignment is perfectly orthogonal) forces the regenerator link to bend in a cyclical fashion as the drive coupling actuates. The regenerator link is thus subject to cyclical stress in a misaligned cryocooler, which leads to material fatigue or catastrophic failure of the connecting rod. But due to real-world manufacturing tolerance issues, it is unfeasible to guarantee that the motor shaft longitudinal axis is perfectly orthogonal to the regenerator piston longitudinal axis. The resulting cyclical bending of the linkage results in rubbing of the expander displacer against the inner cylinder walls, which leads to frictional build-up of heat at the cold end and thus reduced cooling capacity. In addition, the cylinder wall rubbing increases noise significantly.
Accordingly there is a need in the art for improved mechanical cryocooler linkages that enable tightened mechanical tolerances without inducing excessive bending stresses. In addition, there is a need in the art for improved mechanical cryocooler linkages that enable tightened mechanical tolerances while providing increased cooling capacity and noise reduction.